Reading jfjoch/n_images_per_file from aare config raises when the key is
not set and has no default (e.g. on x06da), which aborted data collection
setup. Treat a failed lookup like a missing config client: log a warning
and let JFJoch use its own images-per-file default.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>